U.S. Pat. No. 6,715,287 is incorporated herein by reference and discloses a method and an arrangement for controlling an internal combustion engine having an air system. Here, a physical quantity is determined by means of at least one physical model. This physical quantity characterizes the air system starting from at least one actuating variable and/or at least one measurement quantity which characterizes the state of the ambient air. The physical quantity is not an input quantity of the physical model.